Project Summary Head and neck squamous cell carcinoma (HNSCC) remains a lethal disease despite concerted efforts to improve its diagnosis and treatment. Although smoking, drinking, and HPV infection are closely linked to HNSCC, little is known about how the composition of the microbiome affects HNSCC pathogenesis or its treatment. Currently, most studies linking the microbiome to cancer have focused on gut microbiota, which either contributes locally to colorectal cancer pathogenesis, or more broadly, modulates metabolism and immunity systemically. The oral cavity, which is more directly relevant to the local microenvironment in HNSCC, harbors one of the most complex and diverse microbiomes of all human anatomical sub-sites. However, how either the gut or oral microbiomes shape the local and systemic microenvironment in HNSCC pathogenesis and treatment are completely unknown. Our long-term goal is to determine the functional role(s) and mechanisms of action of the microbiome in HNSCC and to translate these findings into novel preventative and therapeutic strategies. Our central hypothesis is that that dysbiosis arising during HNSCC pathogenesis promotes an immunosuppressive tumor microenviroment that impairs tumor surveillance, accelerates tumor development, and compromises response to HNSCC immunotherapy This hypothesis has been formulated on the basis of publications and preliminary data produced in the applicants' laboratories and will be tested by pursuing three specific aims: 1) Test the hypothesis that an altered microbiome promotes specific stages of HNSCC pathogenesis through generation of metabolites and immune factors that establish an immunosuppressive microenvironment 2) Test the hypothesis that AhR activation by host- and/or microbiota-derived metabolites promotes HNSCC pathogenesis. 3) Test the hypothesis that an altered microbiome compromises HNSCC-directed therapies. Our approach is innovative because it represents a departure from the status quo by utilizing unique experimental mouse models and state-of-art technologies to move beyond observational human studies to delineate the molecular, cellular, and immunological mechanisms induced by the microbiome in HNSCC pathogenesis. The proposed research is significant because it is expected to advance and expand understanding of how the microbiome as a whole, as well as specific microbial species, impacts host immunity in HNSCC development and immunotherapy. Ultimately, such knowledge has the potential to be developed into effective therapies for HNSCC patients, a pressing need given the significant incidence and poor prognosis of this disease.